All of the following are characteristics of anti-Lu^a except which one?

• A. It causes hemolytic disease of the newborn (HDN)
• B. It reacts optimally at room temperature IgM
• C. It causes mixed field reactions
• D. It reacts well in saline

Answer: (A)

Anti-Lu^a is a Lutheran system antibody directed against the Lu^a antigen. It is typically an IgM antibody that reacts best at room temperature and in saline, so you’ll see strong immediate-spin agglutination in saline tests. Mixed-field reactions can occur because Lu^a expression on red cells can be variable, leading to two populations in a test (Lu(a+) and Lu(a−) cells) showing different degrees of agglutination. Reactions well in saline reflect its IgM nature and its ability to agglutinate without added serum or enhancement reagents. HDN is not a typical consequence of anti-Lu^a, since Lu antigens are not consistently expressed on fetal red cells and the antibody is usually IgM, which does not cross the placenta in significant amounts; only if an IgG component is involved would HDN be possible, which is uncommon. So the statement about causing HDN is not a characteristic of anti-Lu^a.